apps/s_socket.c: print only dynamically allocated port in do_server.

For formal backward compatibility print original "ACCEPT" message for
fixed port and "ACCEPT host:port" for dynamically allocated.

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5956)